1. Field of the Invention
The present invention relates to fuel injection equipment of an internal combustion engine, specifically of a diesel engine, which is devised to reduce the generation of nitrogen oxide(NOx) and at the same time to improve mechanical reliability.
2. Description of Related Art
In a diesel engine, the air sucked into the cylinder is compressed in the cylinder and fuel is injected into the compressed air of high pressure and high temperature in the form of spray to self ignite, and the piston is pushed down by the combustion pressure to generate power. It is absolutely necessary to equip the fuel injection equipment to inject the proper amount of fuel into the combustion chamber at proper injection timing.
The structure of conventional fuel injection equipment for injecting fuel into a diesel engine will be explained here with reference to FIG. 13. The drawing shows a fuel injection system using a unit injector 33 in which a fuel injection nozzle part for injecting fuel into the combustion chamber and a plunger part for supplying the highly pressurized fuel to the injection nozzle are integrated. As shown in the drawing, a fuel supply part 10 comprises a fuel tank 11, a fuel supply pump 12, and a volume chamber 13.
The fuel in the fuel tank 11 is pressurized and sent to the volume chamber 13 by the supply pump 12. The pressurized fuel temporarily resides in the volume chamber, then is sent to the plunger part 31 by way of the fuel passage 21 and main electromagnetic valve 41. The fuel is further pressurized in the plunger part 31 and is sent to the injection nozzle part 35 by way of the injection pipe 39 to be injected from the injection holes 38 into the combustion chamber. The redundant fuel not to be injected returns by passing through the injection pipe 39 and overflow pipe 22 by way of the check valve 43 and secondary electromagnetic valve 42 attached to the over flow pipe 22 to the fuel supply part 10. The modes of returning fuel will be described later.
The main electromagnetic valve 41 is attached to the fuel passage 21 to open and close the passage and the secondary electromagnetic valve 42 is attached to the overflow pipe 22 to open and close the pipe passage. The valves 41, 42 are direction control electromagnetic valves of the two-position normally open type, which have an opened position and a closed position. The check valve 43 permits the fuel to flow only from the injection pipe 39 side to the fuel supply part side 10. The opening/closing control operation of the electromagnetic valves 41 and 42 will be described later.
The unit injector 33 is composed of the plunger part 31 and injection nozzle part 35 integrated in an injector body (not shown in the drawing). The plunger part 31 and injection nozzle part 35 are located in series and they are communicated with each other by the fuel injection pipe 39 formed in the unit injector 33.
A roller 51 is attached to the plunger 32 of the plunger part 31. The roller 51 contacts with a cam 52. The cam 52 is driven by the output shaft (crankshaft) of the diesel engine. The plunger 32 reciprocates as the cam 52 rotates. Therefore, if the plunger 32 is lifted by the cam 52 when both the electromagnetic valves 41 and 42 are closed, the fuel compressed by the plunger 32 is sent through the injection pipe 39 to the injection nozzle part 35 from the nozzle 38.
A pressure spring 37 exerts a force on the fuel valve (nozzle needle) 36 in the injection nozzle part 35 to seat it on the nozzle seat. When the force by the pressure of the fuel sent from the plunger part 31 to lift the fuel valve 36 becomes higher than the force of the spring 37, the fuel valve 36 is pushed against the pressure spring 37 to be lifted and the fuel is injected into the combustion chamber in the cylinder in the form of fuel spray.
Next, the injection characteristics by such fuel injection equipment will be explained with reference to FIG. 14 which shows the change of the injection pressure and so on with time (crank angle). FIG. 14(a)˜(f) each shows the following:
FIG. 14(a) . . . injection rate,
FIG. 14(b) . . . fuel valve lift,
FIG. 14(c) . . . fuel injection pressure,
FIG. 14(d) . . . lift of main electromagnetic valve,
FIG. 14(e) . . . lift of secondary electromagnetic valve, and
FIG. 14(f) . . . cam lift.
When the plunger 32 reaches a predetermined lift by the rotation of the cam 52, the main electromagnetic valve 41 is shifted from the open state to the closed state. The pressure of the fuel increases as the plunger 32 is lifted. A spring is incorporated in the check valve 43 attached to the overflow pipe 22, the check valve is opened when the force by the fuel pressure exceeds that of the spring, and the fuel returns to the fuel supply part 10. The fuel valve 36 is lifted more and more as the pressure of the fuel increases resulting in increased injection rate.
When the cam lift of the cam 52 increases further, the secondary electromagnetic valve 42 is shifted from the open state to the closed state. During the period from the perfect closing of the main electromagnetic valve 41 to the opening of the secondary electromagnetic valve 42, a part of the fuel returns through the overflow pipe 22 to the fuel supply part 10 side, so the fuel injection pressure is kept constant. According to the design, the pressure during this period is not constant, it may slightly increase or decrease, however, it is nearly flat.
As the fuel injection pressure during period T1 is flat, the injection rate during this period is suppressed as shown in FIG. 14(a).
When the secondary electromagnetic valve 42 is perfectly closed in the state the main electromagnetic valve 41 is perfectly closed, the fuel injection pressure increases from the flat state, thus the suppression of the injection rate is released and the injection rate increases.
Then, the main electromagnetic valve 41 and secondary electromagnetic valve 42 are shifted from the closed state to the opened state, the fuel injection pressure decreases, and the injection rate decreases to zero.
As the fuel injection rate in the initial part of the injection period, particularly during period T1, can be controlled by controlling opening/closing of the two electromagnetic valves 41 and 42, the fuel is not injected at a dash into the cylinder in the initial period, so the injection quantity in the initial period can be suppressed. As a result, rapid combustion of a large amount of fuel in the initial period of fuel injection is prevented, combustion temperature is suppressed to a low level, and the generation of nitrogen oxide(NOx) is reduced.
A check vale 43 is used in the prior art shown in FIG. 13 and FIG. 14. The check vale 43 comprises movable parts such as a spring and a valve, so mechanical failure has occurred often in use over a prolonged period, which reduces the reliability of the fuel injection equipment.
Further reduction in nitrogen oxide(NOx) by further suppressing the injection rate in the initial injection period is required. However, the prior art has not been able to address such a need.